The present invention pertains to the radio communication art and, more particularly, to a radio/antenna combination.
The radio communication art has developed numerous antenna configurations for use in combination with radio transmitters and/or receivers. Of particular relevance to the instant invention, are antennas developed for use in portable, or hand-held radio transceivers. Two prominent such prior art antennas are the sleeve dipole and the two wire "J" match antenna. A common construction of the sleeve dipole utilizes coaxial cable wherein the outer conductor, or braid of the coax extends back over the cable for an electrical length of one-quarter wavelength at the frequency of interest. The center conductor of the cable extends beyond the folded back joint of the outer conductor for a distance of one-quarter wavelength. Thus, the extended center conductor and outside of the folded back braid comprise a half wavelength dipole. The coaxial line formed by the folded back outer conductor and the outer conductor of the original cable form a short circuited quarter wavelength line thereby creating a choke which tends to limit current to the half wavelength radiator.
A "J" matched antenna is comprised of a half wavelength section fed by a transmission line. The transmission line is generally a quarter wavelength long, thus acting as an impedance transformer to provide a match between the transmitter and the antenna element. When fed by a balanced line, the quarter wave transmission line is generally shorted, with the feed line connected at predetermined taps on the transmission line section. For unbalanced feed conditions, such as coaxial cable, the transmission line is open at both ends, with the feed lines connecting to each of the open ends of the transmission lines.
Tests conducted on both of the above described antenna types have shown that neither is totally suitable for certain applications. For example, in portable communication devices, such as hand held radio transmitters, it is extremely important that the antenna be decoupled from the radio. If surface currents from the antenna exist along the case of the radio two undesired effects result. Firstly, the radiated power, or received energy, of the radio is significantly degraded due to I.sup.2 R losses in the hand, or body of the user. Moreover, in many applications a positive feedbck loop can be established whereby circuitry within the radio is caused to oscillate due to feedback from the antenna element.